1903.] Interferential Form of Elasticity Apparatus. 195 



use of a single screw in each case fitted with a special device to 

 eliminate back-lash. 



(7.) A control apparatus, to enable the observer to modify at will 

 the rapidity with which the bending force is applied, so as to adequately 

 retard the transit of the interference bands and permit of their easy 

 counting. It depends on an exceedingly fine vertical screw, which 

 carries under a bracket at its head an agate plate arranged above a 

 platinum-iridium control-point, similar to the pressure-point, carried on 

 a saddle of the beam midway between the latter and the central fulcrum 

 wedge. The screw is rotated by a diminishing gear, so that a move- 

 ment of the lever handle manipulated by the observer to the extent of 

 3 cm. corresponds to the transit of a single band. 



To prevent flickering of the bands when the control is removed and 

 the whole weight allowed to play, due to earth tremors or other 

 disturbances, an aluminium disc is attached below the balance pan by 

 a short rod and immersed in cedar oil, a platinised counterpoise being 

 .added to that for the pan already on the other side. The slight 

 viscosity of this oil scarcely diminishes the sensibility of the balance, 

 while its resistance to the vertical movement of the horizontal disc 

 renders the bands absolutely steady for the determination of their 

 position when the weight is fully operative. 



VOL. LXXII. 



